Process for the electrolytic production of sodium peroxodisulfate

ABSTRACT

Sodium peroxodisulfate is produced by direct electrolysis by electrolyzing, at 0.3 to 1.2 amperes per square centimeter, a solution containing sodium sulfate and sulfuric acid in such quantity that the solubility of the sodium peroxodisulfate formed amounts to less than 0.6 mole per liter, and maintaining the concentration of sodium sulfate and sulfuric acid substantially constant during the electrolysis.

Unite States Patent [1 1 3,915,816 Rossbcrger Oct. 28, 1975 PROCESS FORTHE ELECTROLYTIC 1,059,809 4/1913 Adolph et al. 204/82 PRODUCTION OFSODIUM 2,281,090 4/1942 Sallevas 204/82 PEROXODISULFATE FOREIGN PATENTSOR APPLICATIONS [75], Inventor: Erwin Rossberger, Hohenrain, 81,4045/1895 Germany 204/82 Germany 205,069 12/1908 Germany 204/82 [73]Assignee: Peroxid-Chemie Gmbl-l, Munich,

Germany Primary ExammerF. C. Edmundson Attorney, Agent, or Firm-Burgess,Dinklage & [22] Filed: Mar. 6, 1974 Sprung [21] Appl. No.: 448,610

[57] ABSTRACT [30] Foreign Application Priority Data Sodiumperoxodisulfate is produced by direct electrol- Sept. 18, 1973 Germany2346945 W y electrolyzing, at to amperes P Square centimeter, a solutioncontaining sodium sulfate and [52] US. Cl. 204/82 lf ric acid in suchquantity that the solubility of the [51] Int. Cl. C25B 1/14; C25B l/28sodium p lfa e f rm m n s o les h n [58] Field of Search 204/82; 156/19mole p liter, and maintaining the concentration of sodium sulfate andsulfuric acid substantially con- [5 6] References Cited stant during theelectrolysis.

UNITED STATES PATENTS 12 Claims, 1 Drawing Figure 880,599 3/1908Teichner et al 204/82 U.S. Patent Oct. 28, 1975 3,915,816

PROCESS FOR THE ELECTROLYTIC PRODUCTION OF SODIUM PEROXODISULFATE Thisinvention relates to the production of sodium peroxodisulfate. Morespecifically, the invention relates to a novel process for theproduction of this material by direct electrolysis.

A number of attempts at the production of sodium peroxidisulfate(hereinafter referred to simply as sodium persulfate) by directelectrolysis have been described in the past. However, the processesinvolved have not been successfully applied in practice. In theelectrolysis of sodium hydrogen sulfate solutions, for example,relatively high current yields of about 80% and more are achieved at thebeginning of the electrolysis, but it is soon observed, i.e., withinseveral hours, that the current yield rapidly diminishes, and may dropto negative values, mathmatically speaking. Various measures which havebeen described in the literature, such as the use of mercury as cathode,have not produced useful results. This discouraging situation has beeninterpreted in part as being due to the action of the Na ion on theanodic oxidation process. Therefore, the prevailing view has been thatdirect electrolysis for the production of sodium persulfate from sodiumsulfate is technically impossible or uneconomical.

Consequently, a four-step process is used for the technical productionof sodium persulfate, which may be described by the following equations:

By electrolysis, an ammonium persulfate solution is formed from ammoniumbisulfate solution with current yields averaging 70 percent, plushydrogen and oxygen. The electrolytic cells used for this purposerequire means at the cathode, such as an asbestos cord wrapping or thelike, to combat the ad-diffusion of the anions so that the reduction ofthe persulfate ions may be limited or prevented. A precipitation of (NHS O in this case is generally undesirable and leads to a considerableincrease in the cell voltage and hence in the energy requirement.

From the ammonium persulfate solution produced in accordance withEquation (1), as free of acid as possible, the salt is obtained incrystalline form by evaporation or precipitation:

The salt is separated by suitable separating units and then dried. Thenit is reacted with sodium hydroxide or caustic soda solution inaccordance with:

This reaction must be performed at reduced pressure with the consumptionof heat, Nl-I and H having to be removed as gas and vapor, respectively.The ammonia that forms must finally be absorbed in an H SO receiver:

and be recycled to the electrolysis process.

The balanced equation of this process may thus be represented asfollows:

Caustic soda solution and sulfuric acid serve as raw materials in thistechnical process. In addition to the electrolysis apparatus requiredfor step (1) of the process, a considerable amount of apparatus isrequired for the remaining steps, mainly for the combined processes (3)and (4). During the process, certain conditions, mainly with regard totemperature and pressure, must be maintained precisely. A transformationof 95 to 98 percent is achieved in step (3), i.e., the losses of activeoxygen are economically acceptable. In this step, every precaution mustbe taken to see that there are no decomposition catalysts present in thesolution. The presence of such materials results in reductions of yieldand also in a hazardous condition: in an extreme case, catalyticdecomposition would result in explosion with catastrophic consequencesfor personnel and equipment.

The invention provides a process for the direct electrolytic productionof sodium persulfate suitable for use on a large commercial scale inwhich the product formed is recoverable without a thermalcrystallization process, i.e., in which a major drawback of prior artprocesses is overcome.

Essentially, the process of the invention comprises the directelectrolytic production by electrolyzing, at 0.3 to 1.2 A/cm 'a solutioncontaining Na SO and H SO in such quantity that the solubility of thesodium peroxodisulfate that is formed will be less than 0.6 mole perliter and the concentration of Na SO and H SO is kept constant duringthe electrolysis.

The process of the invention may be represented by the followingequation:

The knowledge that the concentration of the sodium persulfate dissolvedin the electrolyte must be kept as low as possible in order to limitlosses through reduction of the S 0 at the cathode (reversal of Equation5) has proven to be essential to the successful performance of theelectrolytic process of Equation (5). On the other hand, the anodicoxidation of the sulfuric acid at high HSOf ion concentrations(schematically in accordance with 2 H; 2 W 0,}- 2 e is promoted;surprisingly, however, under the conditions in accordance with theinvention, the anticipated negative effect of the H ions formed inaccordance with Equation (6) in the sense of a hydrolysis of thepersulfate ion as follows:

is hardly perceptible.

For the performance of the electrolysis process a solution is used whichcontains 2.6 to 3.1 moles Na SO per liter and 2.8 to 3.5 moles l-I SOper liter. In this electrolyte, Na S O has a solubility of 0.2 to 0.6mole. During the electrolysis the feedstocks must be added constantly insuch measure that the concentration of acid, sulfate and sodium ionswill remain as constant as possible. Then, after the saturation limit isreached in the startup phase of the electrolysis, newly formed sodiumpersulfate immediately precipitates in crystallized form and is thusremoved from any further reactions. It can best be separatedcontinuously in a partial stream by means of suitable separating units.After washing and drying, a very pure salt is obtained.

To achieve an optimum yield, it has proven desirable to limit the totalamount of dissolved sodium persulfate, since the amount of persulfatethat is exposed to hydrolysis in accordance with Equation (7) and whichis lost to the process will remain relatively small. Accordingly, theamounts of electrolyte used in an electrolysis apparatus are not toexceed approximately 200 liters per kA with reference to the installedcurrent intensity. Preferably, the operating range is 80 to 160 l/kA.

The preferred anode material is pure platinum in the form of sheets,wires or bands, or in the form of a coating on suitable supports. Thesesupports will also serve as a conductor of current to the platinum whichalone has the electrochemical action.

The anodic current density necessary for the anodic oxidation of thehydrogen sulfate ion amounts to at least 0.3 A/cm it may be increased upto 1.2 A/cm if provision is made for sufficient cooling. The electrolytetemperature may amount to as much as about 28C, or even more for shortperiods. The best yields are obtained if the temperature is notsubstantially more than 22C. The addition of polarization-increasingcompounds, such as chloride, borate, cyanide, rhodanide, etc., ispreferred for optimum results. If such compounds are added it isdesirable to keep their concentration constant, too.

The process of the invention will be further explained with the aid ofthe following examples and the drawing:

EXAMPLE 1 An electrolyte, consisting of 3.0 moles Na SO per liter, 3.3moles of H SO per liter and 0.5 g NaCl 0.8 g NaCN per liter, was pumpedby means of a pump (5) through the electrolysis cell l where thechemical process of Equation (5) took place, was separated in container(2) from the electrolysis gas mixture (3) and was recycled intocontainer (4). From the latter a partial stream was fed to thesaturation tank (6) where the raw materials (8) were continuouslymetered in at a rate corresponding to the electrochemicaltransformation. The electrolysis was performed in a flow-through cellwith a current density of 0.5 A/cm at a current drain of 60 A; the timeof stay of the electrolyte in the cell was 0.37 seconds, and itstemperature was C; the electrolyte volume was 8 liters. After about 6hours of operating time, crystallized Na s 0 begain to pretion.

EXAMPLE 2 In the same system as described in Example 1, an electrolyteconsisting of 2.8 moles of Na SO per liter, 3.1 moles of H SO per literand 0.4 g of NaSCN per liter, was electrolyzed at 0.6 A/cm Steadycurrent yields of about 60 percent were produced over the 6-day courseof the experiment. The cell voltage was 5. 1 volts. This shows anelectrolysis power requirement of about 1.9 kWh per kg of sodiumpersulfate.

For comparison, 1.5 to 2.0 kWh/kg must be reckoned for the technicalelectrolysis processes known or commonly used at the present time forthe production of ammonium persulfate. To this must be added the powerrequired for the reactions of Equations (3) and (4), so that the totalconsumption amounts to 3 kWh/kg. The process of the invention,therefore, not only involves substantially simpler procedures but alsomakes possible a very appreciable reduction of power consumption.Furthermore, the invention dispenses with the vacuum apparatus requiredin the known chemical process and the elevated temperatures in the stagecorresponding to Equation (3). From the safety standpoint, too,therefore, it represents an advance in relation to this known technicalprocess.

It will be understood that the specification and examples areillustrative but not limitative of the present invention and that otherembodiments within the spirit and scope of the invention will suggestthemselves to those skilled in the art.

What is claimed is:

1. Process for the direct electrolytic production of sodiumperoxodisulfate from (sodium hydrogen sulfate) a solution containing NaSO and H SO which process comprises electrolyzing in a cell, at acurrent density of from 0.3 to 1.2 amperes per square centimeter, anelectrolytic solution containing said Na SO and H SO in such quantitythat the solubility of the sodium peroxodisulfate formed amounts to lessthan 0.6 mole per liter, and maintaining the concentration of Na SQ, andH 50 constant during the electrolysis.

2. Process as claimed in claim 1 wherein said solution contains from 2.6to 3.1 moles of sodium sulfate per liter and contains at least 0.1 molarexcess of sulfuric acid over sodium sulfate.

3. Process as claimed in claim 1 wherein said solution contains from 2.8to 3.5 moles of sulfuric acid per liter and contains at least 0.1 molarexcess of sulfuric acid over sodium sulfate.

4. Process as claimed in claim 1 wherein the current density is from 0.4to 0.7 amperes per square centimeter.

5. Process as claimed in claim-l wherein said solution also containspotential-increasing material, the concentration of which is maintainedconstant.

6. Process as claimed in claim 5 wherein said potential-increasingsubstance is a chloride, borate, cyanide or rhodanide.

7. Process as claimed in claim 1 wherein the electrolyte solution iscycled into and out of the cell.

perature of the electrolyte solution is maintained at below 28C.

11. Process as claimed in claim 10 wherein said temperature ismaintained between 16 and 22C.

12. Process as claimed in claim 1 wherein the volume of the electrolytesolution does not exceed about 200 liters per kA of installed currentintensity.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 1 3,915,816

DATED October 28, 1975 INVENTOR(S) Erwin Rossberger It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Col. 4, Claim 1, lines 2 and 3, cancel "(sodium hydrogen Y sulfate)". I

I Signed and Scaled this D v 1 Twenty-fourth of August 1976 [SEAL] v v QArrest;

RUTH c.- MASON c. MARSHALL DANN- Anesting Officer Commissioneroj'Par'em: and Trqdemqrks

1. PROCESS FOR THE DIRECT ELECTROLYTIC PRODUCTION OF SODIUMPEROXODISULFATE FROM (SODIUM HYDROGEN SULFATE) A SOLUTION CONTAININGNA2SO4, WHICH PROOCESS C OMPRISES ELECTROLYZING IN A CELL, AT A CURRENTDENSITY OF FROM 0.3 TO 1.2 AMPERES PER SQUARE CENTIMETER, ANELECTROLYTIC SLUTION CONTAINING SAID NA2SO4 AND H2SO4 IN SUCH QUANTITYTHAT THE SOLUBILITY OF THE SODIUM PEROXODISULFATE FORMED AMOUNTS TO LESSTHAN 0.6 MOLE PER LITER, AND MAINTAINING THE CONCENTRATION OF NA2SO4 ANDH2SO4 CONSTANT DURING THE ELECTROLYSIS.
 2. Process as claimed in claim 1wherein said solution contains from 2.6 to 3.1 moles of sodium sulfateper liter and contains at least 0.1 molar excess of sulfuric acid oversodium sulfate.
 3. Process as claimed in claim 1 wherein said solutioncontains from 2.8 to 3.5 moles of sulfuric acid per liter and containsat least 0.1 molar excess of sulfuric acid over sodium sulfate. 4.Process as claimed in claim 1 wherein the current density is from 0.4 to0.7 amperes per square centimeter.
 5. Process as claimed in claim 1wherein said solution also contains potential-increasing material, theconcentration of which is maintained constant.
 6. Process as claimed inclaim 5 wherein said potential-increasing substance is a chloride,borate, cyanide or rhodanide.
 7. Process as claimed in claim 1 whereinthe electrolyte solution is cycled into and out of the cell.
 8. Processas claimed in claim 7 wherein the average residence time of aelectrolyte solution in the cell is 1.0 second or less.
 9. Process asclaimed in claim 6 wherein dispersed solid sodium peroxodisulfate isseparated from the electrolytic solution while being cycled outside ofthe cell.
 10. Process as claimed in claim 1 wherein the temperature ofthe electrolyte solution is maintained at below 28*C.
 11. Process asclaimed in claim 10 wherein said temperature is maintained between 16*and 22*C.
 12. Process as claimed in claim 1 wherein the volume of theelectrolyte solution does not exceed about 200 liters per kA ofinstalled current intensity.